1. Field of the Invention
The present invention relates to apparatus and method for drilling a well into earth formations lying below a body of water, wherein the wellhead equipment of the well is positioned below the surface of the water. The well is drilled from a floating drilling vessel, with a riser conduit connecting the vessel drilling equipment to the wellhead assembly.
2. Description of the Prior Art
An increasing amount of offshore deepwater exploratory well drilling is being conducted in an attempt to locate oil and gas reservoirs. These exploratory wells are generally drilled from floating vessels. As in any drilling operation, drilling fluid must be circulated through the drillbit in order to cool the bit and to carry away the cuttings. This drilling fluid is normally returned to the floating vessel by means of a large diameter pipe, known as a riser, which extends between the subsea wellhead assembly and the floating vessel. The lower end of this riser is connected to the wellhead assembly which is generally located adjacent to the ocean floor, and the upper end usually extends through a centrally located hull opening of the floating vessel. A drillstring extends downward through the riser into earth formations lying below the body of water, and drilling fluids circulate downwardly through the drillstring, out through the drilling bit, and then upwardly through the annular space between the drillstring and the riser, returning to the vessel.
As these drilling operations progress into deeper waters, the length of the riser and consequently its unsupported weight also increases. Since the riser has the same structural buckling characteristics as a vertical column, riser structural failure may result if compressive stresses in the elements of the riser exceed the metallurgical limitations of the riser material. Two separate mechanisms are typically used to avoid the possibility of this cause of riser failure.
Riser tensioning systems are installed onboard the vessel, which apply an upward force to the upper end of the riser, usually by means of cable, sheave, and pneumatic cylinder mechanisms connected between the vessel and the upper elements of the riser.
In addition, buoyancy or ballasting means may also be attached to the submerged portion of the riser. These usually are comprised of syntactic foam elements or individual ballast or buoyancy tanks formed on the outer surface of the riser sections. The ballast or buoyancy tanks are capable of being selectively inflated with air or ballasted with water by utilization of the floating vessel's air compression equipment. Both of these buoyancy devices create upwardly directed forces in the riser, thus compensating for the compressive stresses created by the riser's weight, and thereby preventing riser failure.
Since the riser is fixedly secured at its lower end to the wellhead assembly, the floating vessel will move relative to the upper end of the riser due to wind, wave, and tide oscillations normally encountered in the marine environment.
This creates a problem because the portion of the stationary riser located within the hull opening of the oscillating vessel can contact and damage the vessel, unless it remains safely positioned within the hull opening. For this reason motion compensating equipment incorporated with the riser tensioning system is used to steady the riser within the hull opening, and usually takes the form of pneumatically and/or hydraulically actuated cable and sheave mechanisms connectably engaged between the upper riser elements and the vessel structure, and a flexible coupling located in the riser adjacent the vessel's hull. This equipment allows the vessel to undergo moderate heave, pitch, roll, and sway motions without contacting the upper elements of the riser.
A floating drilling vessel maintains its position over a subsea well by means of a system of mooring lines and anchors, or a system of dynamic positioning thrusters, or a combination of mooring lines and thrusters. Such positioning systems compensate for normal current and wind loading, and prevent riser separation due to the vessel being pushed away from the wellhead location.
All of these systems, however, can only prevent riser compressive failure, separation, or contact with the vessel during normal sea state conditions. The capacity of these systems is exceeded with winds typically over 35 to 40 mph and/or swells over a height of 25 feet. Also, the vessel's dynamic positioning system is subject to failure without warning, which causes the vessel to "drive" off its normal position over the well. Under either of these conditions, measures need to be taken to prevent damage to the vessel and riser.
The riser may be disconnected from the wellhead and then disassembled in sections and stowed on the floating vessel's deck, but the time required for this operation usually exceeds the warning time given by an oncoming storm, and certainly would not be practical in the event of a positioning system failure.
The riser may be disconnected from the wellhead and then disassembled in sections and stowed on the floating vessel's deck, but the time required for this operation usually exceeds the warning time given by an oncoming storm, and certainly would not be practical in the event of a positioning system failure.
The riser may be disconnected from the wellhead assembly and then maintained suspended from the vessel. The vessel with the suspended riser then may remain in the vicinity of the wellhead assembly until conditions permit re-connection to the wellhead, or the vessel may attempt to tow the riser out of the path of an approaching storm. In either situation, once the riser's lower element is released from the wellhead assembly, the riser becomes a vertically oriented submerged vessel with its own oscillatory heave characteristics, or "bobbing" tendencies, typically different than those of the supporting vessel. When the riser, which may be under considerable tension from the tensioning system on the vessel, is released abruptly from the wellhead assembly, the riser will accelerate upward, with the result that the upward movement of the riser often may exceed the displacement limits of the riser tensioning system. Also, when the vessel and riser heave upward, due to the vessel riding the crest of a wave, the riser may continue upward while the vessel is falling downward in a subsequent wave trough. This uncontrolled upward and subsequent downward movement of the riser through the center of the hull opening can exceed the allowable vertical movement and load capacity of the normal motion compensating and tensioning equipment, thereby causing severe damage to the vessel and riser, with attendant risk to crew and vessel.
As described in two related copending applications, both entitled "Drilling Riser Locking Apparatus and Method", filed Apr. 9, 1984, apparatus is disclosed which locks the upper end of the drilling riser to the vessel. This eliminates the vertical and lateral movement of the riser relative to the vessel, obviating the above problem. The disclosed related apparatus is comprised of riser locking apparatus carried within the hull opening of the floating vessel adjacent the bottom of the vessel. The riser locking apparatus is carried at this lower elevation so that the angular displacement of the riser at its upper flexible coupling will not cause the riser, in its displaced position, to contact and damage the vessel's hull. The riser locking apparatus disclosed in both of these copending applications comprises a pair of movable beams that can be moved toward each other, at the closest point of travel engaging the upper elements of the riser. Locking these beams in their closed position effectively locks the upper end of the riser to the vessel.
In both of the copending applications, however, proper alignment of the riser with the locking beams in either the vertical or horizontal plane is necessary before the riser may be locked in position.
In copending application Ser. No. 597,994, vertical movement of the riser must be stopped before the movable beams can be closed. In copending application Ser. No. 597,995, the riser must be held in position in the center of the vessel's moon pool before the riser can be raised up between the movable beams and subsequently be latched in place.
In both situations oscillation of the riser must be dampened by devices other than the locking device, prior to the riser being locked in place. Riser positioning means separate from the oscillation dampening means must also be used. The operation of all of the above position and oscillation dampening equipment requires close coordination and concentration by the vessel's crew, often during times of adverse sea state conditions or in response to unexpected failure of the vessel's directional positioning system.
A device need be developed which combines the riser position and oscillation dampening functions in one device.